The present invention relates to a mask-focusing color picture tube in which a plurality of shadow masks are opposed to a phosphor screen at a small distance therefrom and are insulated from each other to define an electrostatic lens by themselves or with a phosphor screen and, more particularly, to a shadow mask structure in such a color picture tube.
In a color picture tube with a conventional shadow mask, the electron beam utility factor is as low as about 20% due to the presence of the shadow mask, and the brightness of the screen is limited. It is known that the best method to improve brightness is to increase the aperture diameter of the shadow mask and post-focus the electron beams. As a color picture tube which improves brightness, a mask-focusing color picture tube is proposed in which an electrostatic lens is formed in the vicinity of a shadow mask. Such a mask-focusing color picture tube is described in Japanese Patent Disclosure Nos. 79963/1973 and 38580/1976, Japanese Patent Publication Nos. 8261/1972 and 31265/1972, Utility Model Registration Publication No. 40681/1977, and U.S. Pat. Nos. 2,971,117 and 4,112,563.
Among these mask-focusing picture tubes, those which use a single shadow mask require that a voltage applied to a metal-backed phosphor screen must be much higher than a voltage applied to the shadow mask. Therefore, secondary electrons generated from the shadow mask are accelerated to impinge upon the screen, thus reducing the clarity of image and lowering the contrast, which is undesired in practice.
In the other mask-focusing color picture tubes, the electrostatic lenses are formed by predetermined potential differences between a plurality of shadow masks. In these color picture tubes, since the focusing power of the electrostatic lens is weak, a great potential difference must be set between the shadow masks. Then, an arc may occured between the shadow masks, which is a serious problem.
Another type of mask-focusing color picture tube is also known in which grill-shaped shadow masks are arranged to form quadrupole lenses in the apertures of the shadow masks so as to enhance focusing force in one direction. However, the grill-shaped shadow masks are inferior in mechanical strength and formability. Therefore, such a color picture tube is also impractical.
As an improvement over these tubes, a mask-focusing color picture tube with projections in the vicinities of the shadow mask apertures has also been proposed in U.S. Ser. No. 351,882. One of the coinventors of this application is also the inventor of the present invention. The structure of the shadow masks of this tube is shown in FIG. 1.
In two shadow masks 1 and 2 shown in FIG. 1, ridge-like projections 3 and 4 are respectively symmetrically arranged with shadow mask apertures 5 and 6 disposed therebetween. These ridge-like projections 3 and 4 oppose each other and extend in the same direction as that of phosphor stripes coated on the screen (not shown). In the shadow masks of this arrangement, lines of strong electric force are induced from the projections 4 to the projections 3. Therefore, an electrostatic lens of stronger focusing power than that obtainable without the projections may be formed. However, at the peripheries of these shadow masks, the deflected electron beam becomes incident on the surface of the shadow mask at a great incident angle. Therefore, the principal plane of the electrostatic lens formed between the shadow masks is inclined with respect to the optical path of the incident electron beam. The central axes of the electron beams which have passed through the apertures of the shadow masks 1 and 2 largely move with fluctuations in the potential difference between the shadow masks 1 and 2. This state is shown in FIG. 2. For the sake of simplicity, FIG. 2 shows a section, along the plane including axes X and Z, of the parts of the shadow masks 1 and 2 at a given distance on the X axis from the centers O' of the shadow masks 1 and 2.
Referring to FIG. 2, line l connecting the centers of apertures 5 and 6 of the shadow masks 1 and 2 coincides with a central axis m of an incident electron beam 7. Surfaces 8 of the shadow masks 1 and 2 form an angle .theta. with respective to the central axis m of an incident electron beam 7. Thus, as may be shown by the dotted line in FIG. 2, the central axis m of the incident electron beam 7 forms an incident angle ##EQU1## with respect to the principal plane n of an electrostatic lens 9. Therefore, with fluctuations in the focusing power of the electrostatic lens 9 due to fluctuations in the potential difference between the shadow masks 1 and 2, a central axis o of an electron beam 10 which passed through the lens 9 toward a phosphor screen 11 also fluctuates. This phenomenon is well known as the coma aberration of lens. Such fluctuations in the central axis o of the electron beam 10 which has passed through the lens 9 prevents the electron beam from bombarding on the corresponding phosphor stripe and degrades the color purity. In order to solve this problem, the electron beam must be made to become incident perpendicularly to the surface of the shadow mask even at the periphery thereof. However, it is normally difficult to accomplish this, since the deflection center of the electron beam does not generally coincide with the radius of curvature of the shadow mask.